50 mg nitrate per liter, a realistic value in all surface waters? A case study in Limburg (Flanders)

In the context of the EU Nitrate directive, agricultural activities, soil and water quality were monitored in a small catchment in north-east Flanders during 4 years (2012 - 2015, the Horstgaterbeek case). The catchment area is about 130 ha and has an important agricultural activity, with livestock farms and grassland, but also arable crops. It is a flat area with sandy to sandy loam soils, underlying sandy and gravelly strata, and shallow groundwater tables. In the area, a monitoring site for surface water quality, belonging to the MAP monitoring network of the Flemish Environmental Agency, is situated in a small ditch. In this ditch, measured nitrate concentrations strongly exceed the limit value (50 mg NO3- l-1) on a regular base and so far, no improvement is observed. In order to gain a better insight in the N-dynamics in the agricultural parcels, a mechanistic modeling approach of the mineral-N-balance in the soil was developed. In this approach, the different in- and outputs of nitrogen are estimated. The N-mineralization from soil organic matter is calculated based on the potential mineralization rate and takes the temperature and moisture content in the soil into account. The soil moisture content as well as the water drainage rates are calculated with a soil moisture balance model developed by the Soil Service of Belgium. This model takes into account soil characteristics, changes of the groundwater table, local precipitation and parcel-specific evapotranspiration, crop rotation and soil cover. The model was ran for a period of 4 years (2012-2015) and was validated with data from several arable fields within the catchment area. The modeling approach of the N-dynamics in the soil allowed the concerned farmers to gain a better understanding of the different factors (fertilization, crop uptake, mineralization, leaching,…) affecting the nitrate residues and nitrate losses from their agricultural parcels to the groundwater. However, based on the model outputs, it seems very hard if not impossible to meet the nitrate limit value of 50 mg/l in surface water under the current nutrient management, and more importantly in the given hydrogeological situation.